1. Filed of the Invention
The present invention relates to a method and a device for purifying an exhaust gas of an engine.
2. Description of the Related Art
If an air-fuel ratio of an air-fuel mixture in a combustion chamber of an internal combustion engine is referred to as an engine air-fuel ratio, an exhaust gas purifying device for an internal combustion engine with multi-cylinder has been known, in which a three-way catalyst is arranged in an exhaust passage, and the engine air-fuel ratio is controlled to be stoichiometric or rich with respect to the stoichiometric air-fuel ratio. If the engine air-fuel ratio is made lean with respect to the stoichiometric air-fuel ratio, the three way catalyst does not purify nitrogen oxides NOx in the exhaust gas sufficiently, and thus the NOx is emitted to the ambient air. Accordingly, the exhaust gas purifying device mentioned above makes the engine air-fuel ratio stoichiometric or rich, to thereby purify NOx, at the three-way catalyst, as much as possible.
On the other hand, a lower fuel consumption rate is desirable, and thus it is desirable to make the engine air-fuel ratio as lean as possible. However, if the engine air-fuel ratio is made lean, the above-mentioned exhaust gas purifying device cannot purify NOx sufficiently. To solve this problem, Japanese Unexamined Patent Publication No. 4-365920 discloses an exhaust gas purifying device for an internal combustion engine with multi-cylinders, the engine having first and second cylinder groups. The purifying device is provided with; an engine operation control device to continuously make each cylinder operation of the first cylinder group a rich engine operation in which the engine air-fuel ratio is rich, and to continuously make each cylinder operation of the second cylinder group a lean engine operation in which the engine air-fuel ratio is lean; a first exhaust passage connected to each cylinder of the first cylinder group; a second exhaust passage connected to each cylinder of the second cylinder group and different from the first exhaust passage; an NH3 synthesizing catalyst arranged in the first exhaust passage for synthesizing ammonia NH3 from at least a part of NOx in the inflowing exhaust gas; an interconnecting passage interconnecting the first exhaust passage downstream of the NH3 synthesizing catalyst and the second exhaust passage to each other; and an exhaust gas purifying catalyst arranged in the interconnecting passage to react NOx and NH3 flowing therein to each other to thereby purify NOx and NH3 simultaneously. In this exhaust gas purifying device, the fuel consumption rate is reduced by increasing the numbers of the cylinders of the second cylinder group in which the lean engine operation is performed, while purifying NOx by synthesizing NH3 from NOx exhausted from the first group and reacting the NH3 and NOx from the second group.
However, this device requires two, separate exhaust passages, one for the first cylinder group, and the other for the second cylinder group. This complicates the structure of the device, and makes the size of the device larger.
An object of the present invention is to provide a method and a device for purifying an exhaust gas of an engine which can purify the exhaust gas sufficiently with a simple structure.
According to one aspect of the present invention, there is provided a method for purifying an exhaust gas of an engine, comprising, in turn: forming an exhaust gas portion of which an exhaust gas air-fuel ratio is lean, and an exhaust gas portion of which the exhaust gas air-fuel ratio is rich, from the exhaust gas of the engine, alternately and repeatedly; and contacting the exhaust gas portions an NH3 synthesizing catalyst and an exhaust gas purifying catalyst comprised of at least one selected from the group consisted of an NH3 adsorbing and oxidizing (NH3-AO) catalyst and a NOx occluding and reducing (NOx-OR) catalyst, in turn, the NH3 synthesizing catalyst synthesizing NH3 from at least a part of NOx in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and passing NOx in the inflowing exhaust gas therethrough when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, the NH3-AO catalyst adsorbing NH3 in the inflowing exhaust gas therein, and desorbing the adsorbed NH3 therefrom and oxidizing the NH3 when an NH3 concentration in the inflowing exhaust gas becomes lower, and the NOx-OR catalyst occluding NOx in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NOx therefrom and reducing the NOx when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich.
According to another aspect of the present invention, there is provided a device for purifying an exhaust gas of an engine having an exhaust passage, comprising: exhaust gas portion forming means arranged in the engine or the exhaust passage for forming an exhaust gas portion of which an exhaust gas air-fuel ratio is lean, and an exhaust gas portion of which the exhaust gas air-fuel ratio is rich, from the exhaust gas of the engine, alternately and repeatedly; an NH3 synthesizing catalyst arranged in the exhaust passage downstream of the exhaust gas portion forming means, the NH3 synthesizing catalyst synthesizing NH3 from at least a part of NOx in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and passing NOx in the inflowing exhaust gas therethrough when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; and an exhaust gas purifying catalyst arranged in the exhaust passage downstream of the NH3 synthesizing catalyst, the exhaust gas purifying catalyst comprising at least one selected from the group consisting of an NH3 adsorbing and oxidizing (NH3-AO) catalyst and a NOx occluding and reducing (NOx-OR) catalyst, the NH3-AO catalyst adsorbing NH3 in the inflowing exhaust gas therein, and desorbing the adsorbed NH3 therefrom and oxidizing the NH3 when the NH3 concentration in the inflowing exhaust gas becomes lower, and the NOx-OR catalyst occluding NOx in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NOx therefrom and reducing the NOx when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich.
The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.